Trunked communication systems are well know in both the telephone and mobile radio communications art. In the latter system, a plurality of communication units share a limited number of communication resources. Examples of such resources include frequency division multiplex (FDM), frequency pairs, time division multiplex (TDM) time slots, etc. Generally, these communication resources are allocated among the plurality of communication units (e.g., mobile radios, portable radios, and consoles), which communication units require the use of a particular service (e.g., interconnect, group dispatch, and private) from time to time. Service resources are allocated by a communication resource allocator, located at the communication site, to requesting communication units.
FIG. 1 shows a typical trunked communication system 100. Coverage area 101 is serviced by communication site 103. That is, communication units 105, 107 request, and are allocated, service resources via communication site 103. Coverage area 109 represents the reduced coverage available for providing services to radios having less transmit power. For example, communication unit 105 might have a 10-watt nominal transmit power, while communication unit 107 might have only a 1.5-watt transmitter. Accordingly, services can be provided to communication unit 105 throughout larger coverage area 101 than can be provided to communication unit 107--whose coverage area is limited to coverage area 109.
The nominal service duration--an expected time required to complete a call or average call length for a particular service type--varies depending on the type of service, the type of data being exchanged by group members, and even the type of communication unit requesting the service. In a mobile environment, where communication units often travel about the coverage area, the expected service duration largely determines whether or not the requested service call will be completed before the unit travels outside the coverage area. That is, depending on the speed and direction that the mobile unit is traveling at the time the service request is made, the probability of completing the call on that service resource is widely variable. As a result, a communication unit may request, and be allocated, a service resource to initiate a call which cannot be completed before it has traveled outside the coverage area. For example, if the request was to initiate an interconnect voice call, the call will simply be dropped when the communication unit travels out of the coverage area (assuming that the system is not equipped with call-handoff capabilities). In this case, one of the parties would be burdened with the task of re-initiating the call.
Another, perhaps more serious, problem might arise in the instance of transmitting important data, from a terminal, fax machine, or the like. In this case the data may need to be entirely re-transmitted (e.g., re-keyed, resent fax) in order to complete the call. The foregoing examples are used to illustrate the short-comings of prior art trunked communication systems. Of course, the problem presented by an incomplete call may be little more than an annoyance to the user, but it is equally likely to be significant in terms of inefficient resource usage, and the impact on customer satisfaction.
Accordingly, there exists a need for a radio communication system which is not constrained by the aforementioned short-comings of the prior art. In particular, a system which takes into account an estimated probability of completing the call, in determining whether or not to allocate a service resource, would be an improvement over prior art communication systems.